It is common in many fields that a door or gate is attached to a rigid structure for pivoting around the point at which it is attached. To automate the opening and closing of the movable element, a strut is commonly used to further connect the rigid and moveable elements.
The strut is typically both extendable and collapsible, so that it can alter its length to allow a movable element, such as a gate, to close at a controlled rate. In many instances, a linkage is employed to connect the strut to the rigid structure. This linkage is pivotable about both the strut and the rigid structure. Such a system is illustrated in FIG. 1.
FIG. 1 illustrates an upright rigid structure A, attached to the ground G, with a constant angle AG. At one end, a moveable element B, such as a gate, is pivotally connected to rigid structure A. Linkage D is pivotally attached to rigid structure A, and is also connected to moveable element B by strut C. Strut C is pivotally attached to both moveable element B and to linkage D. The pivotal joints between A and B, and between A and D, are represented by angles AB and AD respectively.
In order to close moveable element B, linkage D is rotated with respect to rigid structure A. This reduces the force on strut C and so angle AB is reduced. As the angle AB decreases, due to the continued rotation of linkage D, the force exerted on strut C by element B increases. This force causes strut C to shorten by compression of the piston. If the angle AB is known at the point when Strut C starts to shorten it is possible to control the rate of change of angle AB.
The control of the rate of change of this angle is of interest in a number of fields. To control the descent of moveable element B (which can also be described as controlling the rate of change of angle AB), there are two traditional approaches. Both these approaches implement motorised control of the linkage D to control its positioning with respect to rigid structure A. By varying angle AD the mechanical advantage of strut C is changed so that the combined effect of the compression and change of position result in a controlled descent of moveable element B.
The first approach to controlling moveable element B is to use an open loop control that assumes that the system is unchanging and will always respond to a particular input with the same response curve. Though this is a reasonable assumption while the conditions under which the system is operated are controlled, in an uncontrolled environment these assumptions become invalid. For example, if used outdoors, moveable element B may be loaded with snow or ice, thus changing its effective weight, which will cause strut C to compress at a greater rate than it otherwise would. Additionally, strut C is usually modelled as an ideal spring, which deforms linearly with respect to the applied force, but in reality, the struts are known to have a changing gas pressure as a result of temperature variations, and loss of gas through use of the strut. This has the effect of dynamically changing the spring constant associated with strut C. These variations render the open loop control system inaccurate after moderate exposure to a functional environment.
The second traditional approach is to create a multiple-input-multiple-output (MIMO) control system. To create a MIMO control system a detailed model of the system must be constructed. This model accounts for the weight of moveable element B, the pressure in the piston of strut C and the temperature of the gas in the piston among other factors. This model is then used by a MIMO control system such as a linear quadratic regulator, or a sliding state controller to control the rate of change of angle AB. Though the parameterisation of the system will not necessarily account for the aging of strut C, it is possible to modify the parameterisation model to account for the aging effects, and potential loading of moveable element B on an ongoing basis, using the sensed data to determine the fluctuations in the model parameters. This allows the MIMO control system to accurately control the descent of moveable element B. The drawback to this sophisticated approach is that it requires a high degree of complexity in its implementation. Sensors must be connected to all the elements in the system to measure loading, pressurisation and temperature, along with other variables, so that the parameterisation can be maintained. This is both computationally expensive and impractical to implement when cost conscious decisions are required.
It is, therefore, desirable to provide a method of accurately controlling the descent of a moveable element with respect to a rigid structure without requiring monitoring of the elements to determine a new set of parameterisations to create a complex control system.